As a known vibration isolator such as an engine mount which supports a vibration source such as a vehicle engine or the like so as not to transmit vibrations to a vehicle body side, there is a liquid-sealed type vibration isolator including a first fixture to be fixed to a vibration source side, a second fixture to be fixed to a support side, a vibration-isolating base member provided between these fixtures and made of elastomer, a main liquid chamber having a chamber wall formed partly of the vibration-isolating base member, an auxiliary liquid chamber having a chamber wall formed partly of a diaphragm, and an orifice flow channel configured to communicate these liquid chambers, and configured to carry out a vibration damping function and a vibration insulating function by a liquid flow effect due to the orifice flow channel or by a vibration control effect due to the vibration-isolating base member.
As the liquid-sealed type vibration isolator of this type, there is proposed a type which is provided with a plurality of orifice flow channels which are tuned to different frequencies for accommodating vibrations in a wide range of frequencies, so that the orifice flow channel can be switched.
For example, Patent Document 1 shown below discloses a switchable liquid-sealed type vibration isolator which is configured to be capable of closing a opening of a high-frequency side orifice flow channel using urging means such as a spring. In this document, a negative pressure is used for releasing the opening from the closed state. In other words, a switching chamber which allows selective introduction of an atmospheric pressure and a negative pressure is provided behind the diaphragm, so that the high-frequency side orifice flow channel is brought into a closed state by the urging means by introducing the atmospheric air into the switching chamber, and the high-frequency side orifice flow channel is brought into an opened state by introducing the negative pressure into the switching chamber.
Patent Document 2 shown below discloses a configuration in which a sliding member, so-called a plunger, is provided on a partitioning member between the main liquid chamber and the auxiliary liquid chamber, the plunger is urged by the spring to make the high-frequency side orifice flow channel in the opened state, and the plunger is moved upward and downward by the pressure difference between the both liquid chambers to switch the high-frequency side orifice flow channel between the opened state and the closed state.
Patent Document 3 shown below discloses a configuration in which the high-frequency side orifice flow channel is formed inside an upper fixture, and a second auxiliary liquid chamber is provided upward thereof, a movable membrane is arranged in the second auxiliary liquid chamber, and opening and closing of the high-frequency side orifice flow channel is achieved by the upward and downward movement of the movable membrane.
Patent Document 4 shown below discloses a configuration in which a connecting flow channel which communicates the main liquid chamber and the auxiliary liquid chamber is closed using the urging means such as a leaf spring on the partitioning member on the side of the main liquid chamber. However, according to this document, the connecting flow channel is opened when the interior of the main liquid chamber reaches a pressure equal to or below a predetermined value by an input of impulsive heavy-load vibrations, whereby liquid is leaked from the auxiliary liquid chamber to the main liquid chamber to restrain cavitation, but it is not configured to switch the orifice flow channels tuned into different frequencies.
Patent Document 5 shown below discloses points such that a movable membrane that partitions between the main liquid chamber and an intermediate chamber is configured with a panel-shaped member formed of a rubber elastomer, an elastic deformation of the movable membrane in the panel thickness direction is provided with non-linear spring characteristics to cause the movable membrane to be deformed in an area having low-spring characteristics when idling vibrations are input, and to be deformed up to an area having high-spring characteristics when vibrations having amplitudes larger than the idling vibrations are input in an area having frequencies higher than a resonance frequency of a first orifice flow channel. However, according to this document, it is only means to provide the movable membrane with rigidity depending on the amplitude and improve a damping performance in a large amplitude range, but opening and closing operations of a second orifice flow channel are not performed.    Patent Document 1: Japanese Patent No. 3663482    Patent Document 2: JP-A-2004-003614    Patent Document 3: JP-A-2008-051214    Patent Document 4: JP-A-2007-107712    Patent Document 5: JP-A-2007-046777